A diesel engine system generally comprises one or more combustion chambers which are individually defined by a reciprocating piston inside a cylinder.
The cylinder is provided with one or more intake valves for cyclically opening the combustion chamber towards an intake line for receiving fresh airflow, and with one or more exhaust valves for cyclically opening the combustion chamber towards an exhaust line for discharging the exhaust gases.
The cylinder is also provided with electrically controllable injection means, which are controlled by a microprocessor based controller (ECU), for injecting fuel inside the combustion chamber according to a multi-injection pattern.
During normal engine operation, the multi-injection pattern usually comprises several injections of fuel which follow one another in the time slot between the closure of the intake valves and the instant when the piston reaches its top dead center position (TDC), during the compression stroke.
Such injected fuel burns inside the combustion chamber producing high temperature and pressure gases, whose expansion directly apply force to the piston for moving it towards its bottom dead center position (BDC), in a power stroke which is useful for generating torque at the crankshaft.
The exhaust valves open the combustion chamber nearly when the piston reaches its bottom dead center position, enabling the exhaust gases to flow into the exhaust line during the next exhaust stroke of the piston.
A diesel oxidation catalyst (DOC) is conventionally located in the exhaust line, for degrading residual hydrocarbons and nitric oxides which are formed in the combustion process of the engine and are contained in the exhaust gas flow.
In order to accomplish tighter emission legislation, a diesel particulate filter (DPF) is generally located in the exhaust line downstream the DOC, for capturing and removing diesel particulate matter (soot) from the exhaust gas flow. Usually the diesel particulate filter is joined with the diesel oxidation catalyst forming a single group, which can be located in a closed-coupled configuration or in an under-floor configuration according to the architecture of the exhaust line.
In the closed-coupled configuration, such a group is located near the exhaust manifold of the engine, immediately downstream the turbine of the turbocharger. In the under-floor configuration, such a group is located far from the exhaust manifold, and a diesel oxidation pre-catalyst is usually interposed between the group and the turbine of the turbocharger.
The diesel particulate filters generally comprise a filter body of porous material, with dead-end holes extending into the filter body from opposite sides thereof. In normal operation, exhaust gas enters the dead-end holes from one side of the filter body, and passes through the filter material into the dead-end holes of the other side, whereby the particulate matter carried by the exhaust gas is retained at the surface and in the pores of the filter body. The accumulating particulate matter increases the pressure drop across the filter.
When the pressure drop becomes excessive, it may cause the filter body to crack, rendering the filter ineffective, or it may affect the efficiency of the diesel engine.
In order to avoid excessive clogging of the filter, the particulate matter must be removed when critical amount of it has accumulated in the filter body. This process is generally referred to as regeneration of the diesel particulate filter. Conventionally, regeneration is achieved by heating the DPF to a temperature at which the accumulated particulate matter burns off, leaving the filter body clean again.
It is known to heat the filter by means of a temperature increase of the exhaust gases entering the DPF. This temperature increase (typically up to 630° C.) has to be kept for a certain time (typically 600 seconds) in all possible driving condition (i.e. city driving, highway driving, etc.).
Exhaust gas temperature increase is obtained with a dedicated multi-injection pattern, by means of which an amount of fuel is injected into the combustion chamber after the piston has passed its top dead center position, and the fuel that was injected before is already burnt.
Such late-injected fuel can get a first temperature increase due to fuel combustion inside combustion chamber, and a second temperature increase due to fuel oxidation inside the catalyst (DOC) of the exhaust line.
More particularly, the first temperature increase is achieved by a single injection of fuel which is generally referred to as after-injection. The after injection starts before the exhaust valves opening, and sufficiently near to TDC for the fuel to burn quite completely into the combustion chamber. The combustion of after-injected fuel produces hot gases which are subsequently discharged from the combustion chamber and channeled by the exhaust line to pass through the DPF, whereby the latter is heated.
The second temperature increase is achieved by one or more injections of fuel which are generally referred to as post-injections. Post-injections start sufficiently far from TDC for the fuel to not burn into the combustion chamber, typically after the exhaust valves opening. Therefore, the post-injected fuel is ejected unburnt from the combustion chamber and is channeled by the exhaust line towards the diesel oxidation catalyst (DOC). The diesel oxidation catalyst is effective to oxidize unburnt post-injected fuel, heating the exhaust gases that subsequently pass throw the DPF.
Current strategies for regeneration process are able to achieve the necessary target DPF temperature in most of the engine working condition, which are typically defined in terms of engine speed, engine load and also vehicle speed. However, not all possible engine working condition are actually covered by current regeneration strategies.
For instance, when the engine works in the low-idle condition, current regeneration strategies get a the temperature increase which unlikely can achieve the requested target of temperature upstream DPF (due to very low temperature of the gas upstream DOC), and so the regeneration is not so efficient to completely burn the particulate matter inside the diesel particulate filter in the requested regeneration duration, especially in light application with small displacement engine and especially for under-floor configuration of DPF.
This leads to a regeneration frequency increase and/or to a longer regeneration process, which increase the risk for the regeneration process to be interrupted by driver key-off, and involve many other drawbacks too.
Most common of such drawbacks are: an increased fuel consumption, due to the high amount of post-injected fuel which is used for heating the DPF and which does not generate torque at the crankshaft (since post-injected fuel is oxidized into the DOC); an increased exhaust line thermal stress and an early component ageing (mostly on the Pre Catalyst), due to the fact that the components of the exhaust line are heated for a long time at high temperature; an increased oil dilution, due to the higher amount of post-injected fuel which tends to pass between the piston and the cylinder reaching the engine oil sump; possibility of DPF clogging if regeneration cannot be carried out at all. All these negative effects are typically recognized in and emphasized by specific but very common driving style, especially the city driving style.
At least one aim of the present invention is to improve the current regeneration strategies in order to match the temperature needed for the DPF regeneration in almost all possible engine working conditions, solving or at least positively reducing the above mentioned drawbacks. Another aim of the present invention is to meet the goal with a simple, rational and inexpensive solution. In addition, other aims, desirable features, and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.